A typical memory array, such as a flash memory, includes rows of wordlines situated over that aligned perpendicular to columns of bitlines, which are formed in a substrate. Bitline contacts are formed between some of the wordlines to connect the bitlines to metal lines, which are formed in an interconnect metal layer situated above the wordlines. Since the spacing between wordlines that are situated adjacent to the bitline contacts is greater than the spacing between other wordlines, the bitline contacts cause an irregularity in wordline spacing in the memory array. During wordline fabrication, this irregularity in wordline spacing causes a difficulty in controlling the width of the wordlines situated adjacent to bitline contacts.
In a typical conventional wordline formation process, a hard mask layer is formed over a layer of polycrystalline silicon (polysilicon) and a layer of photoresist is patterned over the hard mask layer to form lines of photoresist. The layer of photoresist can be patterned in a photolithographic process using a mask. The width of the photoresist lines determines the width of subsequently formed wordlines, which are etched in the polysilicon layer. However, the width of the photoresist lines corresponding to wordlines situated adjacent to bitline contacts is difficult to control in the photolithographic process, and becomes increasingly more difficult to control as wordline width is scaled down.
In a conventional approach, a scattering bar can be used on the mask to increase the space between photoresist lines corresponding to wordlines situated adjacent to bit contacts and, thereby, assist in the photolithographic printing of those photoresist lines. However, if the scattering bar is not sufficiently narrower than the wordline, the scattering bar can be printed on the photoresist layer, which is undesirable. Thus, for the above reason, the scattering bar is not useable for printing photoresist lines for very narrow wordlines, such as wordlines having a width of 0.1 micron or less, having irregular spacing.
Thus, there is a need in the art for an effective method of forming wordlines having irregular spacing in a memory array, such as a flash memory array.